1. Field of the Invention
The present invention relates to an image forming apparatus and an image processing method.
2. Description of the Related Art
An inkjet printer has been proposed as an example of an image forming apparatus configured to carry out an image formation by performing recording scan by plural times on the same image region on a recording medium.
In the inkjet printer, a recording head is reciprocated in a main scanning direction, and while a recording medium is conveyed in a sub scanning direction, ink droplets are ejected from a recording head to be impacted on the recording medium for printing an image. In the inkjet printer, depending on errors due to physical factors for printing the image such as errors in features of the respective nozzles and the sheet conveyance amount and deviations in the recording head distance, fluctuations in the direction and the size of the ink droplets, the impact positions, and the like are generated. In a printing operation based on one-time recording scan, the fluctuations directly cause density unevenness and streaks appearing on the print image, which is a cause of degrading the quality of the image.
In view of the above, as a measure for suppressing the generation of such density unevenness and streaks, a multi-pass recording method has been proposed. According to this technology, by combining an image processing with a print control, it is possible to form an image at a high speed while suppressing the decrease in the image quality due to the density unevenness and the streaks.
Hereinafter, the multi-pass recording method will be described in detail with reference to FIG. 12.
In FIG. 12, a recording head 5101 is composed of eight nozzles 5102 for simplifying the description. Ink droplets 5103 are ejected by the nozzles 5102. Typically, in a case where the main scan recording region in a predetermined recording medium is completed by the one-time recording scan, it is ideal that the ink is ejected in a uniform direction with a uniform ejection amount as shown in FIG. 12.
However, as described above, due to the physical factors at the timing of printing, in the printing operation based on the one-time recording scan, the fluctuations in the direction and the size of the ink droplets ejected from the respective nozzles are generated. As a result, in the head main scanning direction, a white background part periodically exists, and on the other hand, dots are overlapped with each other beyond necessity. The congregation of the dots impacted in such a state is sensed as the density unevenness in an array direction of the nozzles. Also, when a misalignment is generated between the recording scans, a joint part between the recording scans is sensed as the streak.
In view of the above, according to the multi-pass recording method, as shown in FIG. 13, the recording scan is performed by the recording head 5201 by plural times (in this example, three times). In the drawing, a recording region in units of four pixels which is the half of eight pixels in the vertical direction is completed by performing the recording scan by two times. In this case, the eight nozzles 5202 in the recording head 5201 are divided into groups of four nozzles on the upper side (upper side nozzle group) and four nozzles on the lower side (lower side nozzle group). The dots recorded by one nozzle in one-time recording scan are obtained by thinning-out the image data by about half in accordance with a predetermined image data array. Then, by embedding about half dots which are the remaining dots at the second scan into the previously formed image, the recording of the four pixel unit region is completed.
Also, according to the two-pass recording method, the first recording scan and the second recording scan mutually complement in accordance with the predetermined array. As an image data array used for this operation (thinning-out mask pattern), in usual cases, such an array is employed as shown in FIG. 14 that a houndstooth check pattern is formed for one pixel each in vertical and horizontal directions. Therefore, in the unit recording region (in this example, the four pixel unit), the print is completed through the recording scan for the first time to print a houndstooth check and the recording scan for the second time to print a reverse houndstooth check. The upper, middle, and lower stages of FIG. 14 respectively illustrate a state where the record is gradually completed in the same region by using the above-described houndstooth check pattern and the reverse houndstooth check pattern. That is, first, as shown in the upper stage of FIG. 14, the recording of the houndstooth check pattern (black circle) is performed on the predetermined region on the recording medium by using the four nozzles on the lower side at the first recording scan. Next, as shown in the middle stage of FIG. 14, at the second recording scan on the relevant region, the paper feed is performed by the four pixels, and the record of the reverse houndstooth check pattern (white circle) is performed by using all the eight nozzles. Furthermore, as shown in the lower stage of FIG. 14, at the third recording scan on the relevant region, the paper feed is performed by the four pixels again, and the houndstooth check pattern is recorded by using the four nozzles on the upper side.
When the multi-pass recording method is carried out, even in a case where the multi-head having the fluctuations shown in FIG. 13 is used, the influence on the recording medium caused by the fluctuations is suppressed by half. Also, when the misalignment between the recording scans is generated, the influence is suppressed by half. For this reason, the density unevenness in the image to be formed is suppressed. Herein, the example in which the printing is completed by performing the recording scans by two times. In general, if the number of the recording scans is increased, the influence caused by the fluctuations or the misalignment can be suppressed. Thus, it is possible to suppress the density unevenness in proportion the number of the recording scans. On the other hand, the printing time is increased in accordance with the number of the recording scans.
For this reason, in a case where the number of the recording scans is desired to be decreased for performing a printing at a high speed, it is difficult to average the fluctuations in the ink droplets and the misalignment between the passes. As compared with the case where the number of the recording scans is not reduced, the density unevenness becomes conspicuous. Therefore, in order to improve the image quality even in the printing with the small number of the recording scans, an appropriate dot arrangement having a feature resistant to the fluctuations in the ink droplets and the misalignment between the passes (hardly decreasing the image quality) should be prepared.
In view of the above, a technology is proposed for creating recording data through thinning-out by using a thinning-out pattern without regularity utilizing random numbers or the like when the recording data corresponding to the respective recording scans is created from the print data. For example, in a case where the printing is performed by performing the recording scan by two times, the thinning-out is performed by using the thinning-out pattern without regularity utilizing the random numbers at the first recording scan, and the thinning-out is performed by using a thinning-out pattern which is obtained by reversing the above-described thinning-out pattern at the second recording scan, thus creating the recording data. As a result, the regularity disappears in the dot configuration as compared with the printing based on the conventional recording scan performed by two times in a related art, and the image quality is improved. However, also as described above, the fluctuations in the ink droplets and the misalignment between the recording scans are generated. As the complementary relation is established by performing the thinning-out by using the mask pattern between the respective recording scans, if the fluctuations in the ink droplets and the misalignment between the recording scans are caused, overlapped dots and periodic white background parts are formed, which tend to be sensed as the density unevenness. In particular, the misalignment between the recording scans interferes the dot pattern, and the density unevenness and the streaks appear as inappropriate patterns over the entire scan.
Therefore, it is necessary to prevent the dot pattern from interfering any dot patterns created in the print data of the respective recording scans in a case where the misalignment between the recording scans is generated. However, it is difficult to obtain a mask pattern which prevents the interference of the dot patterns for any input images.
In view of the above, to cope with such a problem, a technology is proposed for distributing the respective pixel values of the relevant image data the stage of the multi-value image data for each recording scan through a method of dividing the pixel values at a constant ratio or a method of dividing the pixel values while randomly changing a rate. Furthermore, quantization is performed on the respectively distributed multi-value data, and the image corresponding to the recording scan with the restrained complementary relation is generated. Through these processings, the degree of dependence in the change of the image density with respect to the fluctuations in the ink droplets and the misalignment between the passes is decreased, and the image quality is improved.
However, in a case where the image corresponding to the respective recording scans is generated through the above-described method, as shown in FIG. 15, in some cases, the dot arrangement is disproportionate between the passes, and the dots are overlapped with each other. For this reason, the streaks and the unevenness are sensed on the print image due to the dot arrangement between the recording scans in some cases. In a case where the pixel values of the multi-value image data are distributed at a constant ratio, depending on the dot arrangement created as a result, the dot patterns of the respective pass images are interfered between the recording scans, which tends to be sensed as the streaks and the unevenness on the print image. On the other hand, in a case where the pixel values are distributed at a random rate, a local density change is caused when the misalignment is generated between the passes at the time of the printing, which tends to be sensed as the unevenness. In order to further improve the image quality, the fluctuations in the ink droplets, a method of suppressing the interference of the dot patterns and the density change when the misalignment is generated in particular between the passes is demanded.